🍼 Conception Calculator: Estimate When You Conceived
By Shihab Mia · Reviewed by ToolNimba Review Team, Health content reviewed for accuracy · Updated 2026-06-26
This conception calculator gives an estimate for general information only and is not professional medical advice. Conception cannot be pinned to one exact day, so confirm any pregnancy dating with an ultrasound and your healthcare provider.
This is an estimate for general information only, not a medical diagnosis. Conception spans several days and cannot be pinned to one date. Confirm any pregnancy dating with an ultrasound and your healthcare provider.
This conception calculator estimates roughly when conception occurred and shows a likely conception window of a few days. Work either way: enter the first day of your last menstrual period (LMP) and your cycle length, or enter your estimated due date and let the tool count backward. Because the real date depends on when you ovulated and how long sperm survived, the answer is an informed estimate rather than a single fixed moment.
What is the Conception Calculator?
Conception is the moment a sperm fertilizes an egg, and it happens around ovulation. In a textbook 28 day cycle, ovulation falls about 14 days after the first day of the last menstrual period (LMP), so conception is usually estimated as the LMP plus 14 days. Cycles that run longer or shorter shift ovulation with them. A reliable rule is that ovulation happens about 14 days before your next period, so conception lands roughly (cycle length minus 14) days after the LMP. That is why this calculator lets you set your own cycle length instead of assuming 28 days for everyone.
When you only know your estimated due date, the math runs in reverse. Pregnancy is clinically dated as about 280 days (40 weeks) from the LMP, and since conception happens roughly two weeks after the LMP, conception sits about 266 days (38 weeks) before the due date. The calculator simply subtracts 266 days from the due date to find an approximate conception date, and subtracts 280 days to recover the matching LMP.
Why show a window instead of one day? An egg survives only about 12 to 24 hours after ovulation, but sperm can live in the reproductive tract for up to about 5 days. Intercourse several days before ovulation can still result in pregnancy, so the actual fertilization can land a few days away from the calculated estimate. The few day window in the results reflects that biology, which is also why a tool like this can never be used to judge paternity or set a legal date.
The results also report gestational age and trimester for context. Gestational age is counted from the LMP, the standard clinical convention, so it runs about two weeks longer than the time since conception. Weeks 1 to 12 are the first trimester, weeks 13 to 26 the second, and weeks 27 to 40 the third. These figures help you place the conception estimate inside the wider pregnancy timeline, but an early ultrasound remains the most accurate way to date a pregnancy.
When to use it
- Getting a rough idea of when conception likely happened after a positive pregnancy test.
- Working back from a clinic or ultrasound due date to an approximate conception date.
- Checking which week or trip might line up with the conception window out of curiosity.
- Cross-checking the conception estimate that a pregnancy app or provider gave you.
How to use the Conception Calculator
- Choose your mode: from your last menstrual period (LMP) or from your estimated due date.
- Enter the single date that applies, and if using LMP, set your average cycle length.
- Read the estimated conception date and the likely conception window below.
- Use the copy button to save the dates, then confirm important dating with your provider.
Formula & method
Worked examples
Your last period started on 1 March and your cycle is the usual 28 days.
- Offset from LMP = cycle length - 14 = 28 - 14 = 14 days
- Conception = 1 March + 14 days = 15 March
- Window = 15 March - 3 days to 15 March + 1 day
- Window = 12 March to 16 March
Result: Estimated conception around 15 March, with a likely window of 12 to 16 March.
You only know your estimated due date, which is 1 December.
- Conception = due date - 266 days
- 1 December minus 266 days lands on about 10 March of the same year
- Estimated LMP = due date - 280 days, about 24 February
- Window = 7 March to 11 March around the conception estimate
Result: Estimated conception around 10 March, with a likely window of 7 to 11 March.
How the estimate is calculated from the date you know
| You know | Estimate method | Typical result |
|---|---|---|
| LMP, 28 day cycle | LMP + 14 days | Conception about 14 days after LMP |
| LMP, longer cycle | LMP + (cycle - 14) days | Conception more than 14 days after LMP |
| LMP, shorter cycle | LMP + (cycle - 14) days | Conception fewer than 14 days after LMP |
| Estimated due date | Due date - 266 days | Conception about 38 weeks before due date |
Key timing facts behind the estimate
| Event | Approximate timing |
|---|---|
| Pregnancy length from LMP | About 280 days (40 weeks) |
| Pregnancy length from conception | About 266 days (38 weeks) |
| Ovulation in a 28 day cycle | About day 14 |
| Egg viability after ovulation | About 12 to 24 hours |
| Sperm survival in the body | Up to about 5 days |
| First trimester | Weeks 1 to 12 from LMP |
Common mistakes to avoid
- Treating the estimate as an exact day. Conception is calculated, not observed. Because sperm can survive several days and ovulation timing varies, the true date can be a few days off. Rely on the window, not the single date.
- Using the LMP method with an irregular cycle. The LMP plus 14 rule assumes a regular cycle. If your cycles vary or you do not track them, the estimate can be well off, so an early ultrasound dates a pregnancy far more reliably.
- Confusing the conception date with the LMP date. Providers date pregnancy from the LMP, which is about two weeks before conception. So your "weeks pregnant" number is not the time since conception, it runs roughly two weeks longer.
- Using it to determine paternity. A conception estimate spans several days and is not precise enough to identify a father. Only a DNA test can establish paternity, and this tool should never be used for that purpose.
Glossary
- Conception
- The moment a sperm fertilizes an egg, which usually happens around the time of ovulation.
- LMP
- Last menstrual period, the first day of your most recent period. Pregnancies are commonly dated from this date.
- Ovulation
- The release of an egg from the ovary, typically about 14 days before the next period is due.
- Gestational age
- How far along a pregnancy is, counted from the LMP, so it is about two weeks more than the time since conception.
- Conception window
- The span of days within which conception likely occurred, used because the exact day cannot be known.
- Trimester
- One of three stages of pregnancy: the first runs weeks 1 to 12, the second weeks 13 to 26, and the third weeks 27 to 40.
Frequently asked questions
How does the conception calculator work?
If you enter your last menstrual period (LMP), it adds about 14 days (adjusted for your cycle length) because ovulation, and so conception, usually happens around then. If you enter a due date, it subtracts 266 days, since conception is roughly 38 weeks before the estimated due date.
How accurate is the estimated conception date?
It is an estimate. Ovulation timing varies and sperm can survive up to about 5 days, so the real conception date can differ from the calculated one by several days. Treat the window as the realistic answer rather than the single date.
Can I find out exactly when I conceived?
No method can pin conception to a single day after the fact. An early ultrasound is the most reliable way to date a pregnancy, but even that gives a range rather than an exact moment of conception.
Why is conception about two weeks after my last period?
In a typical 28 day cycle, ovulation happens around day 14, which is roughly two weeks after the first day of your last period. Conception occurs around ovulation, so it lands about two weeks after the LMP.
My cycle is not 28 days, can I still use this?
Yes. With the LMP method you can enter your average cycle length, and the tool estimates ovulation as about 14 days before your next period, or (cycle length minus 14) days after the LMP. For irregular cycles the estimate is less reliable, so confirm with your provider.
Can this calculator tell me who the father is?
No. The estimate covers a window of several days and is not precise enough to determine paternity. Only a DNA test can establish who the father is, so please do not rely on this tool for that.
Sources
- Calculating a Due Date , Johns Hopkins Medicine
- Trying to Conceive , Office on Women's Health, U.S. Department of Health and Human Services